I have three 120 gallon heaters

Got a question for ya. This is Bill Julian. (Plumber) How would I pipe 3 hot water heaters so I don't create a reverse loop and they draw equally together. I have 3 120 gallon heaters. I'm told by the inspector that I am over working one heater the system is not equally drawing. Said something about a reverse loop and the tees have to situated correctly.

All the colds got in series all the hots come out in series I have the hot 3/4 tee to fixtures near the end of the 3rd heater.

Sounds like you have eather A.O. Smith cyclone or STATE water heaters
go to hotwater.com web site,
for A.O. Smith and they will show diagram of the
proper way to pipe as they are supposed to be per, mfg's spec's and plumbing code also you can call 1-800-527-1953 for tech, support ! !

Do not forget to make sure you have the proper sized thermal expansion tanks
installed for all three heaters

As Mark asked "what are you doing anyway, is this some sort of hotel??? " How many fixtures are in this building and how many people are in it? Three Gal heaters is insane. If it is a hotel or a place that needs real fast recovery why not use a 200 Gal storage tank a A.O. Smith Burkay boiler with a recirculate pump. I had a restaurant in Chicago that has this set up installed. I had to replace the 20 year old storage tank, once I refilled the system and fired it up they had 180 degree water in 12 minutes. Another option is tankless heaters.

Here is a couple pictures showing tankless heaters installs one is gas other is electric.

With more information about the job like if your using gas fired heaters and the BTU's of each heater, or if they are electric, and how many fixture units is the demand on this place. Then we can give you a more accurate answer on how to install what you want.

You get either the State #27200 or A O Smith #70415 three tank manifold kits, or you get their specifications and make your own. Once you see the prices you will know why this is not a "by the seat of your pants" task.